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Back-up alternator

glenn654

Well Known Member
For those who have installed a back-up alternator running from the vacuum pump drive, do you still think it has been a worthwhile addition considering expense, weight, conplexity?

Glenn Wilkinson
 
Perhaps a better question...

perhaps a better question:

Has anyone that has a back-up alternator ever had an in-flight situation that required the it?

We have twin batteries and a single alternator, and there are scant few failure modes that do not have me safely on the ground using up the capacity of one or the other battery. I never could justify the back-up alternator, so I didn't put one in, but I'd love to hear the "this is the situation where you must have it" story.
 
I've used mine once in 4 1/2 years. Back in March we were flying down to Key West to meet friends. I was on an IFR flight plan but the wx was clear and beautiful. About 30 minutes out of Key West, the primary alternator failed. The VP-200 did it's thing and brought the backup (SD-20) online just as advertised.
This is definitely not a "must have it" story. In this particular situation, I could have simply shut down non-essential electrical and used battery power to get on the ground.
I debated whether to go the dual battery or dual alternator route and decided to go dual alternator. A second battery would have added as much or more weight as the alternator. Not much complexity with the VP-200, so the dual alternator choice was right for me.
 
I'm still building but chose the SD8 over dual batteries for several reasons:

-Costs for the SD8 will be cheaper in the long run over maintaining 2 batteries
-I would be willing to bet the SD8 (2.9 lbs) with wiring and regulator is lighter than a second battery (PC680 15 lbs) with large cables and relays
-Less complex to test before flying. The backup battery "should" provide a known amount of amp/hr but how long since the last capacity test?

I almost installed a backup battery, but after thinking about future upgrade ability, maintaining and testing the second battery I chose the SD8.
 
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Weight was a big factor in my decision to go with a standby alternator vs two batteries. With a second battery you will be covered in the event of an alternator failure, in that it will safely let you fly for an hour or so. However if you are more than an hour or so away from home, with electronic ignition like I have, you will still have to get the alternator fixed before you continue your travels. With a stand by alternator you just flip a switch, fly home and fix the problem there.
 
For those who have installed a back-up alternator running from the vacuum pump drive, do you still think it has been a worthwhile addition considering expense, weight, conplexity?

Glenn Wilkinson

Yes. Mine comes on all the time. I routinely exceed the ~50amps of my primary.
 
With dual EI, a reliable source of power was a priority for me, my B&C 40A primary alt is still going strong so I've never needed the back-up SD8.

Electrical power is like fuel, better to have it and not need it then need it and not have it.

Tom Martin nailed it pretty well.
 
used it once so far

I have the B&C SD8 and have used it once in the two years I have had the plane. The main alternator failed just as I was landing on a short trip for lunch. it was a vfr day but I would have been stranded without it. Flew home using just the SD8 and it handled the reduced load just fine.

I use the backup battery on every flight and often when I do not fly but want to upgrade databases or just try something out on the GPS or GRT systems. I can play around until it completely dies and still not have to worry about my main battery starting the engine and getting everything charged back up.

In summary, IMHO the BU alternator is really nice to have (especially for ifr), the 2nd battery is a must have. Plus I love the 2.5 hour battery in the Garmin 696 as a completely standalone last resort.
 
used it once so far

I have the B&C SD8 and have used it once in the two years I have had the plane. The main alternator failed just as I was landing on a short trip for lunch. it was a vfr day but I would have been stranded without it. Flew home using just the SD8 and it handled the reduced load just fine.

I use the backup battery on every flight and often when I do not fly but want to upgrade databases or just try something out on the GPS or GRT systems. I can play around until it completely dies and still not have to worry about my main battery starting the engine and getting everything charged back up.

In summary, IMHO the BU alternator is really nice to have (especially for ifr), the 2nd battery is a must have. Plus I love the 2.5 hour battery in the Garmin 696 as a completely standalone last resort.
 
Not wanting to get into the discussion about overall system reliability (with dual alternators) as I don't have that arrangement and can't comment.

But let me ask you this - if you have dual alts, are you going to alter your trip if one of your alternators fails??
 
Not wanting to get into the discussion about overall system reliability (with dual alternators) as I don't have that arrangement and can't comment.

But let me ask you this - if you have dual alts, are you going to alter your trip if one of your alternators fails??


If enroute ifr or vfr and the main fails, I would continue to destination. If on the ground at destination, I would not launch ifr without the main alternator but would fly Vfr with only the BU working.
 
I went the dual-alternator route because I deleted the engine-driven fuel pump and went with dual electric pumps, so my engine is electrically dependent. Dual alternators gives me more options than dual batteries, each alternator can supply the full electrical load of the plane. In the extremely unlikely event that BOTH alternators go tango-uniform I can shed all load except the fuel pumps and the battery will run that for about an hour. My Dynon Skyview has it's own internal backup battery good for about that same period of time so I get to keep one of the screens without touching the electrical budget on the ships main battery.

In the case of losing the primary in flight and operating on the backup, since I'm dependent on power for fuel I would continue to my planned destination and swap out the primary there - I would not launch in IFR weather or over rough terrain or water without having both alternators functional.
 
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Yes

Jim Berry
RV-10

To elaborate on my initial answer, I lost my primary alternator while IFR(due to a spun bearing). My SD20 backup came online without any input from me, just as designed. Completed the flight to our destination. Didn't bother to load shed, as the SD20 will power everything in my glass panel, except heated pitot and landing lights. If we had lost the backup, we would still have approx. 45 minutes to get on the ground.

Jim Berry
RV-10
 
Yes. Mine comes on all the time. I routinely exceed the ~50amps of my primary.

Is the wiring diagram of the SD8 any different if using it as a "suppplimental" alternator vs a "back-up" alternator? If so does anyone have a link to show how different?
 
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Safety??

A backup alternator is a great safety feature, especially when mounted on engine number two. I know, I know... I suppose, if your primary alternator can't handle the total amps a second might help, but, I would first sell that one and get another that can handle the amps - very little weight gain and maybe a bit less money lost than adding alternator #2. However, if your ignition system requires external voltage, then that would be an argument for a second alternator and maybe a backup battery or two or three.

I did lose my alternator this summer. I knew I lost it because the field breaker opened, efis voltage alarms went off, and volts dropped below 12. I turned off all unnecessary electrical stuff , then turned around and flew 2 1/2 hours home on the battery. Easy peasy.

And now, for the cockpit voice recorder, "ymmv".
 
I got to use my backup alternator in the heat of battle yesterday for the first time, I was testing my oxygen system at 17,500 right over my home strip in CAVU conditions. I heard a thump-WHAP from the engine compartment and about 2 clock seconds later I hear Dynon in my headset saying "Voltage". I have the verbal alarm set to go off if it drops below 13.0 volts, which it will do almost immediately when the alternator goes offline. I took a moment to assess if anything else on the EMS page was going where it shouldn't be, then pulled the breaker on my main and flipped the field switch on for the alternate, the voltage immediately came up where it should be.

I checked my pulse-ox one more time (O2 saturation fine running the Mountain-High EDS - pulse was WAY high, go figure) and terminated my oxygen test, pulled power and dropped it onto the runway. Spent the next 3 hours replacing the alternator belt and a subsequent test flight showed the primary alternator working just fine again.
 
To elaborate on my initial answer, I lost my primary alternator while IFR(due to a spun bearing). My SD20 backup came online without any input from me, just as designed. Completed the flight to our destination. Didn't bother to load shed, as the SD20 will power everything in my glass panel, except heated pitot and landing lights. If we had lost the backup, we would still have approx. 45 minutes to get on the ground.

Jim Berry
RV-10

My experience is almost identical to Jim's. Two primary alternator problems in 8 years and over 900 hours and both times the SD20 picked up the load as programmed in the VP-200, and allowed me to continue to destination. Both times were a ground wire on the back of the alternator that looked fine, but had an internal break at the connector. First time I could not find it. Cleaned up all the connections tightened the ground-to-case bolt that was a little loose, but not really loose enough to cause a problem. But, that "fixed" it for about 40 hours. Second time I got lucky and figured out the problem. My system is set up to load shed, but, as Jim says, my SD20 comfortably powers everything but pitot heat, landing lights, and, of course, air conditioning. Very happy with mine.
 
The ability to keep going

I've been thinking hard about adding an SD8. It's not just the in flight failure I'm thinking of, but the belief that I could continue a multi-leg x-country trip by minimizing power consumption after losing the primary alternator.

The 32ah battery I have now weighs about 24lbs, but I can and have flown hours (VFR) on the battery alone after a fan belt failure. I would go with the 4lb Earth x, if I had the backup alternator and end up with a lighter plane. John
 
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